Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven wind turbine” or “direct drive wind turbine”) or through the use of a gearbox.
An important auxiliary system generally provided on wind turbines is a pitch system. Pitch systems are employed for adapting the position of a wind turbine blade to varying wind conditions by rotating the blade along its longitudinal axis. In this respect, it is known to rotate a wind turbine blade in such a way that it generates less lift (and drag) when the wind speed increases. This way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blade towards its vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance, or to prevent damage to the wind turbine under extremely high wind speed conditions.
Many pitch systems comprise an electric motor in the hub which drives an actuating gear. Said actuating gear (pinion) meshes with an annular gear provided on the wind turbine blade to set the wind turbine blade into rotation. It is also possible however, that the annular gear is provided on the hub, whereas the electric motor and actuator are mounted on the blade. Yet other actuating mechanisms, such as e.g. involving hydraulic actuators, are also known.
It is further known to provide an individual pitch system (comprising e.g. a separate motor and separate control) for each wind turbine blade of a rotor. It is also known to provide a common pitch system wherein the pitch angle of the blades is the same for all blades on a rotor. Such a common pitch system may comprise a single motor or may comprise a plurality of motors, one for each blade.
A common control strategy of a pitch system in a variable speed wind turbine is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed (for example approximately 4 m/s-15 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine. Above the nominal speed (for example from approximately 15 m/s-25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. When the wind turbine is not operating, the blades may assume a vane position (e.g. at or around 90° pitch angle) to minimize the loads on the blades. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design.
These blade pitch systems usually comprise a controller of the blade's position which is powered by means of a main power line. When the controller fails or in case of grid loss, the blade pitch system may result inoperable and the related blade and/or the wind turbine may be damaged. Therefore, it is important that control of operation of the blade pitch system is maintained under all or almost all circumstances.
Different prior art systems aimed at improving the security and the reliability of blade pitch systems are known. Some of said prior art systems are based on a backup configuration comprising batteries, which are commonly used when DC motors with collectors are employed as pitch drives. However, this configuration generally cannot reliably control the speed of the pitch system, since said speed may depend on e.g. the state of charge of the batteries.
Some other prior art systems introduce redundant elements in a way that if a main element fails, a secondary equivalent element may assume the role of the main element. For example, U.S. Pat. No. 7,717,673B2 discloses a redundant and fail-safe blade pitch system of a wind turbine including at least one blade pitch drive and at least two power control modules for controlling the blade pitch drive. The power control modules are connected to the blade pitch drive by a switching unit which allows an alternative connection between the blade pitch drive and any of the power control modules.
When the redundant and fail-safe blade system of U.S. Pat. No. 7,717,673B2 comprises only one blade pitch drive and at least two related power control modules, a switching mechanism based on 2-way switches is required and there is always at least one unused power control module. When the redundant and fail-safe blade system comprises several blade pitch drives related to several power control modules, the switching mechanism based on 2-way switches becomes quite complex.